1. Field of the Invention
The present invention relates to an external resonator type light emitting system.
2. Related Art Statement
It has been generally utilized a semiconductor laser of Fabry-Perot (FP) type including an optical resonator constructed with an active layer and mirrors provided on both end faces of the active layer. However, according to such FP type laser, light is oscillated at a wavelength satisfying conditions for oscillating standing waves. The longitudinal mode thus tends to be of multi mode, and the oscillating wavelength is changed particularly when a current or temperature is changed, resulting in a change of optical intensity.
Therefore, for the purpose of optical communication or gas sensing, it is required a laser capable of single mode oscillation and with good stability of wavelength. It has been thereby developed a distributed feed-back (DFB) laser and a distributed reflection (DBR) laser. According to these laser systems, diffraction gratings are provided in a semiconductor material to oscillate light having only a specific wavelength utilizing the dependency of the gratings on wavelength.
According to the DBR laser, concaves and convexes are formed on a surface of a waveguide extended from a waveguide of the active layer to provide a mirror utilizing Bragg reflection for realizing a resonator (Patent Document 1; Japanese Patent Publication No. S49-128,689A; Patent Document 2; Japanese patent Publication No. S56-148,880A). Since diffraction gratings are provided on both ends of the optical waveguide layer according to the laser, light emitted from the active layer is propagated through the optical waveguide layer, and a part of the propagating light is reflected by the diffraction gratings, returned into a current injection part and then amplified. As light component having only a single wavelength is reflected at a specific direction from the diffraction grating, the wavelength of the laser light is made constant.
Further, as the application, it was developed an external resonator type semiconductor laser including a diffraction grating as a part separated from the semiconductor to form an external resonator. Such type of laser provides one having stability of wavelength, stability of temperature and controllability. The external resonator includes a fiber Bragg grating (FBG) (Non-patent document 1) and a volume hologram grating (VHG) (Non-patent document 2). Since the diffraction grating is composed of a member separated from the semiconductor laser, it is characteristic that its reflectance and length of the resonator can be independently designed. And, since it is not affected by temperature rise due to heat generation caused by current injection, it is possible to further improve the stability on wavelength. Further, the temperature dependency of refractive index of the semiconductor is different, it is possible to improve the stability on temperature by designing the refractive index together with the length of the resonator.
According to Japanese Patent document 6 (Japanese Patent Publication No. 2002-134,833A), it is disclosed an external resonator type laser utilizing a grating formed in a waveguide composed of quartz glass. It aims at providing a frequency-stable laser suitably used in environment under which room temperature is considerably changed, (for example, up to 30° C. or higher) without a temperature controller. It is further described to provide a laser free from temperature dependency with mode hopping prevented and without dependency of oscillating frequency on temperature.
According to Patent document 7 (Japanese Patent Publication No. 2010-171252A), it is disclosed an external resonator type laser including an optical waveguide composed of a core layer made of SiO2, SiO1-xNx (x represents 0.55 to 0.65) or Si and SiN, and a grating formed in the optical waveguide. This is an external resonator type laser capable of maintaining the oscillation wavelength constantly without precise temperature control. For this, it is indispensable that the temperature dependence of the reflection wavelength of a diffraction grating (temperature coefficient of the Bragg reflection wavelength) is made low. It is further described that the longitudinal mode of the laser oscillation is further made multi mode to realize stability of power.
According to Patent document 8 (Japanese patent No. 3667209B), it is disclosed an external resonator type laser utilizing a grating formed in an optical waveguide made of quartz, InP, GaAs, LiNbO3, LiTaO3 or polyimide resin. According to this, a reflectance at a light emitting face of a semiconductor laser as a light source is an effective reflectance Re (substantially of 0.1 to 38.4%) and the longitudinal mode of the laser oscillation is made multi mode, so that stability of power can be realized.